1. Field of the Invention
The invention relates to a photovoltaic system with a photovoltaic generator, which comprises multiple parallel strings of series-connected photovoltaic modules, wherein the strings have a positive pole and a negative pole, between which is present a predetermined string voltage over the number of series-connected photovoltaic modules, and with an inverter whose DC input is connected to the two poles and whose AC output can be connected to a supply grid. The invention also relates to a method for operating such a photovoltaic system.
2. Description of the Background Art
A photovoltaic system is known from EP 2 086 020 A2, which corresponds to U.S. Pat. No. 8,138,411, and which is incorporated herein by reference.
When designing a photovoltaic system, care must be taken to ensure that the maximum permissible voltage (Uz) between the positive pole and the negative pole on the DC side of the inverter is not exceeded. The reason for this is that an exceedance results in destruction of the inverter and the part of the photovoltaic modules at which a voltage above a permissible voltage is present.
For this reason, the photovoltaic system customarily is designed such that even in the most unfavorable case of a no-load condition, the string voltage, which is then referred to as the no-load voltage or open-circuit voltage (UL), remains below the maximum permissible string voltage (Uz). In a design with a plurality of parallel-connected strings, the maximum number of strings is determined by the output of the inverter to which the strings are connected. In this context, the inverter can be designed for a DC input voltage of approximately 900V to 1000V.
In an implementation of the photovoltaic system (PV system) with 11 photovoltaic modules (PV modules) in each string, each of which has 120 photovoltaic cells (PV cells), a total of 1320 cells are thus connected in series with one another. A voltage of 0.75V is present at each cell in the no-load condition, resulting in a string voltage of 990V. This value is below the maximum voltage of 1000V specified by the manufacturers of the PV modules.
During operation of the PV system, the open-circuit voltage of the PV cells drops to an operating voltage of approximately 0.5V so that a voltage of 660V is present between the ends of the conventional strings. If the PV system should be taken off the grid unexpectedly, for example due to a short circuit in the supply cable, then the voltage abruptly jumps to the said 990V, which is not critical for the PV modules and the PV system. However, if a higher voltage is present, this can lead to destruction of at least some PV modules and of the inverter as well as the entire PV system.
Now, on the one hand it is desirable, especially with regard to new types of PV modules with relatively high rated voltages and open-circuit voltages, to operate the PV modules and also the inverters at a higher voltage than 660V in normal operation, ideally at the maximum permissible voltage of 1000V. For better utilization of the dielectric strength of the cabling, generally 1000V, it is additionally desirable to increase the number of PV modules per string in order to utilize the 1000V voltage during operation of the PV system. This is not readily possible, however, since a voltage of approximately 1500V in the no-load condition would then lead to the destruction of the PV modules and the inverter as well as the system.
To avoid these impermissibly high voltages, it is known from EP 2 101 391 A2 to arrange a short-circuit switch between the positive pole and the negative pole that short circuits the poles in the event of an impermissibly high voltage between them. In addition, it is known from EP 2 086 020 A2, mentioned earlier, and from DE 20 2006 008 936 U1 to fix the positive pole or the negative pole at a fixed, permissible voltage of, for example, the said 1000V and to allow the PV system to float up or down from this voltage in operation, which is referred to as floating.
This measure is not possible for PV systems with a free floating voltage. In PV systems with a free floating voltage of the positive pole and of the negative pole, voltages of, e.g., plus (+) 600V to minus (−) 600 volts occur with respect to a virtual ground. Virtual ground here means that the strings are not connected to ground at any point. However, if the center of the string is placed at ground, then corresponding voltages of (+) 600V and (−) 600V with respect to the grounded center of the string would be present at the positive and negative poles, respectively.
For such PV systems, it is possible to provide a switch between the center of the string and ground that is closed in the event of a ground fault and actually grounds the center of the string. As a result, then, only voltages of up to 600V occur at the PV modules. However, this measure entails substantial cabling effort, since the center of every single string must be reachable through the switches. Moreover, when the modules known as TCO modules are used, corrosion problems occur, because the edge of the modules is eroded on account of cathode discharge.